-----------------------------------------------------------------------------
-- JSON4Lua: JSON encoding / decoding support for the Lua language.
-- json Module.
-- Author: Craig Mason-Jones
-- Homepage: http://json.luaforge.net/
-- Version: 0.9.20
-- This module is released under the The GNU General Public License (GPL).
-- Please see LICENCE.txt for details.
--
-- USAGE:
-- This module exposes two functions:
--   json.encode(o)
--     Returns the table / string / boolean / number / nil / json.null value as a JSON-encoded string.
--   json.decode(json_string)
--     Returns a Lua object populated with the data encoded in the JSON string json_string.
--
-- REQUIREMENTS:
--   compat-5.1 if using Lua 5.0
--
-- CHANGELOG
--   0.9.20 Introduction of local Lua functions for private functions (removed _ function prefix).
--          Fixed Lua 5.1 compatibility issues.
--   		Introduced json.null to have null values in associative arrays.
--          json.encode() performance improvement (more than 50%) through table.concat rather than ..
--          Introduced decode ability to ignore /**/ comments in the JSON string.
--   0.9.10 Fix to array encoding / decoding to correctly manage nil/null values in arrays.
-----------------------------------------------------------------------------
-----------------------------------------------------------------------------
-- Imports and dependencies
-----------------------------------------------------------------------------
local math = require('math')
local string = require("string")
local table = require("table")

local base = _G

-----------------------------------------------------------------------------
-- Module declaration
-----------------------------------------------------------------------------
local json = {}

-- Public functions

-- Private functions
local decode_scanArray
local decode_scanComment
local decode_scanConstant
local decode_scanNumber
local decode_scanObject
local decode_scanString
local decode_scanWhitespace
local encodeString
local isArray
local isEncodable

-----------------------------------------------------------------------------
-- PUBLIC FUNCTIONS
-----------------------------------------------------------------------------
--- Encodes an arbitrary Lua object / variable.
-- @param v The Lua object / variable to be JSON encoded.
-- @return String containing the JSON encoding in internal Lua string format (i.e. not unicode)
function json.encode(v)
    -- Handle nil values
    if v == nil then
        return "null"
    end

    local vtype = base.type(v)

    -- Handle strings
    if vtype == 'string' then
        return '"' .. json.encodeString(v) .. '"' -- Need to handle encoding in string
    end

    -- Handle booleans
    if vtype == 'number' or vtype == 'boolean' then
        return base.tostring(v)
    end

    -- Handle tables
    if vtype == 'table' then
        local rval = {}
        -- Consider arrays separately
        local bArray, maxCount = json.isArray(v)
        if bArray then
            for i = 1, maxCount do
                table.insert(rval, json.encode(v[i]))
            end
        else -- An object, not an array
            for i, j in base.pairs(v) do
                if json.isEncodable(i) and json.isEncodable(j) then
                    table.insert(rval, '"' .. json.encodeString(i) .. '":' .. json.encode(j))
                end
            end
        end
        if bArray then
            return '[' .. table.concat(rval, ',') .. ']'
        else
            return '{' .. table.concat(rval, ',') .. '}'
        end
    end

    -- Handle null values
    if vtype == 'function' and v == null then
        return 'null'
    end

    base.assert(false, 'encode attempt to encode unsupported type ' .. vtype .. ':' .. base.tostring(v))
end

--- Decodes a JSON string and returns the decoded value as a Lua data structure / value.
-- @param s The string to scan.
-- @param [startPos] Optional starting position where the JSON string is located. Defaults to 1.
-- @param Lua object, number The object that was scanned, as a Lua table / string / number / boolean or nil,
-- and the position of the first character after
-- the scanned JSON object.
function json.decode(s, startPos)
    startPos = startPos and startPos or 1
    startPos = json.decode_scanWhitespace(s, startPos)
    base.assert(startPos <= string.len(s), 'Unterminated JSON encoded object found at position in [' .. s .. ']')
    local curChar = string.sub(s, startPos, startPos)
    -- Object
    if curChar == '{' then
        return json.decode_scanObject(s, startPos)
    end
    -- Array
    if curChar == '[' then
        return json.decode_scanArray(s, startPos)
    end
    -- Number
    if string.find("+-0123456789.eE", curChar, 1, true) then
        return json.decode_scanNumber(s, startPos)
    end
    -- String
    if curChar == [["]] or curChar == [[']] then
        return json.decode_scanString(s, startPos)
    end
    if string.sub(s, startPos, startPos + 1) == '/*' then
        return json.decode(s, json.decode_scanComment(s, startPos))
    end
    -- Otherwise, it must be a constant
    return json.decode_scanConstant(s, startPos)
end

--- The null function allows one to specify a null value in an associative array (which is otherwise
-- discarded if you set the value with 'nil' in Lua. Simply set t = { first=json.null }
function json.null()
    return null -- so json.null() will also return null ;-)
end
-----------------------------------------------------------------------------
-- Internal, PRIVATE functions.
-- Following a Python-like convention, I have prefixed all these 'PRIVATE'
-- functions with an underscore.
-----------------------------------------------------------------------------

--- Scans an array from JSON into a Lua object
-- startPos begins at the start of the array.
-- Returns the array and the next starting position
-- @param s The string being scanned.
-- @param startPos The starting position for the scan.
-- @return table, int The scanned array as a table, and the position of the next character to scan.
function json.decode_scanArray(s, startPos)
    local array = {} -- The return value
    local stringLen = string.len(s)
    base.assert(string.sub(s, startPos, startPos) == '[',
        'decode_scanArray called but array does not start at position ' .. startPos .. ' in string:\n' .. s)
    startPos = startPos + 1
    -- Infinite loop for array elements
    repeat
        startPos = json.decode_scanWhitespace(s, startPos)
        base.assert(startPos <= stringLen, 'JSON String ended unexpectedly scanning array.')
        local curChar = string.sub(s, startPos, startPos)
        if (curChar == ']') then
            return array, startPos + 1
        end
        if (curChar == ',') then
            startPos = json.decode_scanWhitespace(s, startPos + 1)
        end
        base.assert(startPos <= stringLen, 'JSON String ended unexpectedly scanning array.')
        object, startPos = json.decode(s, startPos)
        table.insert(array, object)
    until false
end

--- Scans a comment and discards the comment.
-- Returns the position of the next character following the comment.
-- @param string s The JSON string to scan.
-- @param int startPos The starting position of the comment
function json.decode_scanComment(s, startPos)
    base.assert(string.sub(s, startPos, startPos + 1) == '/*',
        "decode_scanComment called but comment does not start at position " .. startPos)
    local endPos = string.find(s, '*/', startPos + 2)
    base.assert(endPos ~= nil, "Unterminated comment in string at " .. startPos)
    return endPos + 2
end

--- Scans for given constants: true, false or null
-- Returns the appropriate Lua type, and the position of the next character to read.
-- @param s The string being scanned.
-- @param startPos The position in the string at which to start scanning.
-- @return object, int The object (true, false or nil) and the position at which the next character should be
-- scanned.
function json.decode_scanConstant(s, startPos)
    local consts = {
        ["true"] = true,
        ["false"] = false,
        ["null"] = nil
    }
    local constNames = {"true", "false", "null"}

    for i, k in base.pairs(constNames) do
        -- print ("[" .. string.sub(s,startPos, startPos + string.len(k) -1) .."]", k)
        if string.sub(s, startPos, startPos + string.len(k) - 1) == k then
            return consts[k], startPos + string.len(k)
        end
    end
    base.assert(nil, 'Failed to scan constant from string ' .. s .. ' at starting position ' .. startPos)
end

--- Scans a number from the JSON encoded string.
-- (in fact, also is able to scan numeric +- eqns, which is not
-- in the JSON spec.)
-- Returns the number, and the position of the next character
-- after the number.
-- @param s The string being scanned.
-- @param startPos The position at which to start scanning.
-- @return number, int The extracted number and the position of the next character to scan.
function json.decode_scanNumber(s, startPos)
    local endPos = startPos + 1
    local stringLen = string.len(s)
    local acceptableChars = "+-0123456789.eE"
    while (string.find(acceptableChars, string.sub(s, endPos, endPos), 1, true) and endPos <= stringLen) do
        endPos = endPos + 1
    end
    local stringValue = 'return ' .. string.sub(s, startPos, endPos - 1)
    local stringEval = base.load(stringValue)
    base.assert(stringEval, 'Failed to scan number [ ' .. stringValue .. '] in JSON string at position ' .. startPos ..
        ' : ' .. endPos)
    return stringEval(), endPos
end

--- Scans a JSON object into a Lua object.
-- startPos begins at the start of the object.
-- Returns the object and the next starting position.
-- @param s The string being scanned.
-- @param startPos The starting position of the scan.
-- @return table, int The scanned object as a table and the position of the next character to scan.
function json.decode_scanObject(s, startPos)
    local object = {}
    local stringLen = string.len(s)
    local key, value
    base.assert(string.sub(s, startPos, startPos) == '{',
        'decode_scanObject called but object does not start at position ' .. startPos .. ' in string:\n' .. s)
    startPos = startPos + 1
    repeat
        startPos = json.decode_scanWhitespace(s, startPos)
        base.assert(startPos <= stringLen, 'JSON string ended unexpectedly while scanning object.')
        local curChar = string.sub(s, startPos, startPos)
        if (curChar == '}') then
            return object, startPos + 1
        end
        if (curChar == ',') then
            startPos = json.decode_scanWhitespace(s, startPos + 1)
        end
        base.assert(startPos <= stringLen, 'JSON string ended unexpectedly scanning object.')
        -- Scan the key
        key, startPos = json.decode(s, startPos)
        base.assert(startPos <= stringLen, 'JSON string ended unexpectedly searching for value of key ' .. key)
        startPos = json.decode_scanWhitespace(s, startPos)
        base.assert(startPos <= stringLen, 'JSON string ended unexpectedly searching for value of key ' .. key)
        base.assert(string.sub(s, startPos, startPos) == ':',
            'JSON object key-value assignment mal-formed at ' .. startPos)
        startPos = json.decode_scanWhitespace(s, startPos + 1)
        base.assert(startPos <= stringLen, 'JSON string ended unexpectedly searching for value of key ' .. key)
        value, startPos = json.decode(s, startPos)
        object[key] = value
    until false -- infinite loop while key-value pairs are found
end

--- Scans a JSON string from the opening inverted comma or single quote to the
-- end of the string.
-- Returns the string extracted as a Lua string,
-- and the position of the next non-string character
-- (after the closing inverted comma or single quote).
-- @param s The string being scanned.
-- @param startPos The starting position of the scan.
-- @return string, int The extracted string as a Lua string, and the next character to parse.
function json.decode_scanString(s, startPos)
    base.assert(startPos, 'json.decode_scanString(..) called without start position')
    local startChar = string.sub(s, startPos, startPos)
    base.assert(startChar == [[']] or startChar == [["]], 'decode_scanString called for a non-string')
    local escaped = false
    local endPos = startPos + 1
    local bEnded = false
    local stringLen = string.len(s)
    repeat
        local curChar = string.sub(s, endPos, endPos)
        if not escaped then
            if curChar == [[\]] then
                escaped = true
            else
                bEnded = curChar == startChar
            end
        else
            -- If we're escaped, we accept the current character come what may
            escaped = false
        end
        endPos = endPos + 1
        base.assert(endPos <= stringLen + 1, "String decoding failed: unterminated string at position " .. endPos)
    until bEnded
    local stringValue = 'return ' .. string.sub(s, startPos, endPos - 1)
    local stringEval = base.load(stringValue)
    base.assert(stringEval,
        'Failed to load string [ ' .. stringValue .. '] in JSON4Lua.decode_scanString at position ' .. startPos .. ' : ' ..
            endPos)
    return stringEval(), endPos
end

--- Scans a JSON string skipping all whitespace from the current start position.
-- Returns the position of the first non-whitespace character, or nil if the whole end of string is reached.
-- @param s The string being scanned
-- @param startPos The starting position where we should begin removing whitespace.
-- @return int The first position where non-whitespace was encountered, or string.len(s)+1 if the end of string
-- was reached.
function json.decode_scanWhitespace(s, startPos)
    local whitespace = " \n\r\t"
    local stringLen = string.len(s)
    while (string.find(whitespace, string.sub(s, startPos, startPos), 1, true) and startPos <= stringLen) do
        startPos = startPos + 1
    end
    return startPos
end

--- Encodes a string to be JSON-compatible.
-- This just involves back-quoting inverted commas, back-quotes and newlines, I think ;-)
-- @param s The string to return as a JSON encoded (i.e. backquoted string)
-- @return The string appropriately escaped.
function json.encodeString(s)
    s = string.gsub(s, '\\', '\\\\')
    s = string.gsub(s, '"', '\\"')
    s = string.gsub(s, "'", "\\'")
    s = string.gsub(s, '\n', '\\n')
    s = string.gsub(s, '\t', '\\t')
    return s
end

-- Determines whether the given Lua type is an array or a table / dictionary.
-- We consider any table an array if it has indexes 1..n for its n items, and no
-- other data in the table.
-- I think this method is currently a little 'flaky', but can't think of a good way around it yet...
-- @param t The table to evaluate as an array
-- @return boolean, number True if the table can be represented as an array, false otherwise. If true,
-- the second returned value is the maximum
-- number of indexed elements in the array.
function json.isArray(t)
    -- Next we count all the elements, ensuring that any non-indexed elements are not-encodable
    -- (with the possible exception of 'n')
    local maxIndex = 0
    for k, v in base.pairs(t) do
        if (base.type(k) == 'number' and math.floor(k) == k and 1 <= k) then -- k,v is an indexed pair
            if (not json.isEncodable(v)) then
                return false
            end -- All array elements must be encodable
            maxIndex = math.max(maxIndex, k)
        else
            if (k == 'n') then
                if v ~= table.getn(t) then
                    return false
                end -- False if n does not hold the number of elements
            else -- Else of (k=='n')
                if json.isEncodable(v) then
                    return false
                end
            end -- End of (k~='n')
        end -- End of k,v not an indexed pair
    end -- End of loop across all pairs
    return true, maxIndex
end

--- Determines whether the given Lua object / table / variable can be JSON encoded. The only
-- types that are JSON encodable are: string, boolean, number, nil, table and json.null.
-- In this implementation, all other types are ignored.
-- @param o The object to examine.
-- @return boolean True if the object should be JSON encoded, false if it should be ignored.
function json.isEncodable(o)
    local t = base.type(o)
    return (t == 'string' or t == 'boolean' or t == 'number' or t == 'nil' or t == 'table') or
               (t == 'function' and o == null)
end

return {
    encode = json.encode,
    decode = json.decode
}
